I have both filters set at 125hz. The box is sealed and is approx 1sq ft in size. The same box AND amp were used for both speakers. The smaller speaker was wired at 4ohms but this larger sub is wired at 1ohm.
The MTX is a mono amp but it has 4 terminals for convenience. I have the speaker wired in parallel and I'm using the + from one "channel" and the - from the other "channel".
I can listen to a song and my mid-high is NO louder than my door speakers. Literally you wouldn't even think I had a sub in here. Then when it hits the mid-low it sounds "ok". But when it hits the REALLY low notes (not in normal music) it almost throws the sub out of my box and distorts horribly.